This invention relates to the fabrication of integrated-circuit devices and, more particularly, to techniques for achieving improved mask-to-wafer alignment during manufacture of such devices.
X-ray lithography is being actively explored as a promising approach for achieving fine-line patterning of devices. An attractive way of realizing high-precision mask-to-wafer alignment in an X-ray patterning system involves the use of zone plates on the mask and wafer, as described, for example, in U.S. Pat. No. 4,037,969 issued to M. Feldman and A. D. White.
Illustratively, mask and wafer zone plates utilized for alignment purposes are illuminated by a collimated laser beam. Advantageously, the angle of illumination is chosen to be off-normal relative to the surfaces of the mask and wafer, as described in U.S. Pat. No. 4,326,805 issued to M. Feldman, A. D. White and D. L. White. As a result of this off-normal inclination, uniform magnification errors (such as those resulting from variations in the temperature of the wafer) can be detected, and the errors can be corrected by changing the mask-to-wafer separation.
In practice, however, it has been observed that the intensity and in some cases even the location of the centroid of the light spot formed by a laser-beam-illuminated zone plate changes as the mask-to-wafer spacing is varied. Such intensity changes typically complicate the alignment process. And, of course, any changes in the location of the centroid of the imaged light spot inevitably cause actual alignment errors to occur.
Accordingly, efforts have been directed by workers in the art at trying to understand the basis for the aforespecified deleterious phenomena. It was recognized that these efforts, if successful, might lead to improved alignment techniques in which the specified phenomena are eliminated or at least minimized. In turn, the availability of such improved alignment techniques could provide a basis for making higher-quality fine-line devices in a more reliable and less expensive way.